Stablecoins are cryptocurrencies whose price is controlled by active or automatic pricing mechanisms with the aim of achieving low value deviations relative to a particular fiat currency, a combination of different currencies, or other assets.
In other words: Stablecoins aim to achieve price stability (which is why they are called "stable-"coins) and are, in general, considered to be a lot less volatile than other cryptocurrencies (even though there are exceptions - see for example here or here).
This price stability can be achieved via different methods, among them are:
Reserve-based backing: this method uses an underlying asset to stabilize the price of the coin. Examples are fiat-backing (using fiat currencies as the underlying asset), commodity-backing (using a standardized commodity) or cryptocurrency-backing (using another coin as the underlying asset of the stablecoin).
All of the stablecoins currently available via Brighty's earning vault feature are fiat-backed or fiat-pegged stablecoins, deriving their value directly from being tied to a fiat currency (like USD, EUR, or CHF).
Seigniorage-style/algorithmic mechanisms: this method utilizes algorithms to control the stablecoin's supply, in a manner that is similar to a central bank's approach (by increasing or decreasing the amount of fiat currency in circulation).
Stablecoins based on this approach are generally considered a less popular form of stablecoin. Unlike stablecoins backed by the value of an underlying asset, the value of algorithmic stablecoins is controlled by supply and demand (as well as the particularities of the underlying algorithm).
While stablecoins do have their detractors, they are generally considered to be among the safest, most predictable and most reliable of the many different types of cryptocurrencies available. This is especially true for reserved-back stablecoins.
The Bank of International Settlements even lists the possible merits of stablecoins as enhancement of anti-money laundering efforts, operational resilience, customer data protection, financial inclusion, tax compliance, and an overall improvement of the global state of cybersecurity (due to the cryptographic element involved).